private void inicialize() {
    firebaseAuth = FirebaseAuth.getInstance();
    authStateListener = new FirebaseAuth.AuthStateListener() {
        @Override
        public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
            FirebaseUser firebaseUser = firebaseAuth.getCurrentUser();
            if (firebaseUser != null) {
                Log.w(TAG, "onAuthStateChanged - Logueado");

            } else {
                Log.w(TAG, "onAuthStateChanged - Cerro sesion");
            }
        }
    };
}

private void signIn(String email, String password) {
    firebaseAuth.signInWithEmailAndPassword(email, password).addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
        @Override
        public void onComplete(@NonNull Task<AuthResult> task) {
            if (task.isSuccessful()){
                Toast.makeText(LoginActivity.this, "Autenticacion correcta", Toast.LENGTH_SHORT).show();
                Intent intent = new Intent(LoginActivity.this, MainActivity.class);
                startActivity(intent);
                finish();
            } else {
                Toast.makeText(LoginActivity.this, "Autenticacion incorrecta", Toast.LENGTH_SHORT).show();

            }
        }
    });
}

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_login);

    etUsuario = (EditText) findViewById(R.id.etUsuario);
    etPassword = (EditText) findViewById(R.id.etPassword);

    Button btnIngresar = (Button) findViewById(R.id.btn_ingresar);
    Button btnAbrirCrearCuenta = (Button) findViewById(R.id.btnAbrirCrearCuenta);

    //Inicializamos firebase
    inicialize();

    //Al presionar Boton de Ingresar se ejecuta la funcion de Firebase
    btnIngresar.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            //Validamos los inputs
            String email = etUsuario.getText().toString().trim();
            String password = etPassword.getText().toString().trim();

            if (TextUtils.isEmpty(email)) {
                Toast.makeText(LoginActivity.this, "Ingrese un usuario", Toast.LENGTH_SHORT).show();
                return;
            }

            if (TextUtils.isEmpty(password)) {
                Toast.makeText(LoginActivity.this, "Ingrese una contrasena", Toast.LENGTH_SHORT).show();
                return;
            }

            signIn(etUsuario.getText().toString(), etPassword.getText().toString());
        }
    });